Improve connection pool metrics by adding redis instance label

https://dashboards.gitlab.net/explore?schemaVersion=1&panes=%7B%22m3t%22:%7B%22datasour[…]22from%22:%22now-1h%22,%22to%22:%22now%22%7D%7D%7D&orgId=1

Currently we can’t tell which Redis instance does the gitlab_connection_pool_available_count is referring to

This ticket was created from INC-2673 and was automatically exported by incident.io 🔥